Energy Storage : Groundbreaking electrode for potassium ion batteries

In this panorama, the development of economically feasible emerging battery technologies based on alternative, earth-abundant, elements, is thus highly desirable.
Potassium-ion batteries could represent a viable substitute to Lithium-ion technology in a large-scale green economy. However, the key problem preventing the success of the K-ion technology is linked to the low efficiency of cathode materials.

Recently, the group of prof. Stanislav Fedotov from the Skoltech Center for Energy Science and Technology (Moscow, Russia) developed a novel titanium-based K-ion fluoride-phosphate material with the perspective to set an important milestone in the design of future storage systems based on this metal.
In this work, published in Nature Communications, this material, KTiPO₄F, was found to adopt a KTiOPO4-(KTP)-type orthorhombic crystal structure that boosts the Ti⁴⁺/Ti³⁺transition to extraordinarily high electrode potentials approaching 3.6–3.7 V vs. K⁺/K with unprecedented stability at high discharge rates. Understanding the structural evolution of the positive electrode during its charge/discharge is extremely important to explain this unique electrochemical performance. With access to X-ray powder diffraction at the MCX beamline the structural transformation was successfully studied in operando (see data shown in Figure 1). 

According to the diffraction data, the structural transformation of KTiPO₄F starts with a two-phase transition leading to a lower (monoclinic) symmetry. The further K+deintercalation tentatively proceeds via a solid-solution mechanism, as no changes in symmetry are observed, and is followed by another two-phase transition. The potassium depleted material was charged up to 4.2 V reaching the K_0.2TiPO₄F composition without undergoing amorphization, preserving the integrity of the polyhedral framework.
Compared to isostructural compounds, KTiOPO₄ and KVPO₄F, the new material contains slightly larger interstitial voids resulting in the splitting of K ions over several energetically equivalent sites, which ultimately promotes the ionic diffusion.
The work by S. Fedotov et al. shows that the titanium redox activity traditionally considered as “reducing” can be upshifted to near-4V electrode potentials thus offering a playground to design sustainable and cost-effective titanium-containing positive electrode materials with promising electrochemical characteristics.
Overall, this earth-abundant, easily scalable, and thermally stable KTiPO₄F material reveals exciting electrochemical performance outperforming many benchmarked potassium-ion cathode materials.